RMagick and Tempfile and StringIO

I’ve got a file upload that uploads an image to a server. That image is
then manipulated via rmagick.

I’ve only just found that for files less than 10KB in size, that the
object is not a Tempfile, but a StringIO. If it’s a Tempfile (e.g.
image file is over 10KB) the site works fine, but for small images, it

Thing is, that I can’t figure out how to get a StringIO into RMagick to
create an imagemagick image, so that I can manipulate it.

Can anyone help please?


I had EXACTLY this problem when I started using RMagick.

What I do is always call f.read on whatever I get back. For instance,
my form has this in it:

<% form_for :avatar, @avatar,
            :url => { :action => 'create_avatar', :id => @toon },
            :html => { :multipart => true } do |f| %>

  <p><label for="avatar_image">Image</label><br/>
  <%= f.file_field :image_file %></p>

  <%= submit_tag "upload" %>
<% end %>

This means my image goes into "avatar_file" in my controller. I then
call something like this in toon_controller:

  def create_avatar
    @avatar = @toon.avatars.create(params[:avatar])

I think the f.rewind and f.read trick here are the key. From my avatar model:

  def image_file=(f)
    self.image_binary = f.read

  def image_binary=(data)
    imglist = Magick::Image.from_blob(data)
    img = imglist.first
    if img.columns > 75 || img.rows > 75
      img.crop_resized!(75, 75, Magick::NorthGravity)
    img.format = 'PNG'
    self.image = Base64::encode64(img.to_blob)
    self.format = 'image/png'
    self.width = img.columns
    self.height = img.rows

  def image_binary

What this mess does is hide my 'image' field. 'image' is always
SQL-safe Base64 encoded, while image_binary is the undecoded version.
I won't let people store more than the correct size in my database, so
I adjust it on input and just base64 decode it on output. There are
times where I want the actual base64 version (email springs to mind)
but usually I just refer to image_binary and send that.


Michael Graff wrote:

I had EXACTLY this problem when I started using RMagick.

Brilliant. Just what I needed. Sorted it now.